In making measurements in a well borehole with measuring devices supported on a wireline suspended in the borehole, errors are encountered in making depth measurements. Depth measurement errors can arise from a multitude of factors. Several of these factors are discussed in U.S. Pat. Nos. 4,117,600 and also 4,179,817. These two U.S. Patents particularly describe particular errors which may arise in such a system. One error not discussed is the error arising from stretch of the wireline and in particular stretch which occurs during ordinary operations subject to snagging. By way of background, a wireline is subject to stretch which is ordinarily a relationship primarily dependent on wireline length and tension on the wireline. However, during use, the tool supported on the wireline may snag or drag for a variety of reasons and thereby vary the tension. When tension is varied, the amount of stretch will vary. Even worse, the tool at the end of the wireline may snag; when this occurs, the wireline handling apparatus at the surface will continue to retrieve the wireline, tension will increase, and the tool will eventually snap free. When that occurs, the tool will typically jump rapidly to relax the overstressed wireline and overshoot its equilibrium position. The wireline (made of resilient material) acts as a long coil spring which oscillates the tool up and down, a motion superimposed on the linear rate of retrieval. Thus, the tool movement can be very irregular for a interval. This tendency to overshoot after snagging during retrieval creates an error in depth of the measuring instrument.
The present apparatus is a system which provides stretch corrected measurements. In addition to that, it also provides a quality indication to be recorded simultaneously with the data recorded during operation of the logging tool. This quality indication is recorded as a function of depth to thereby indicate those portions of the logged data which may be suspect because they were recorded at a location where tool movement was erratic as a result of snagging, dragging, or overshooting on release. This is particularly helpful in determining whether or not the formation data or depth is highly reliable. The interpretation of the formation data can then be enhanced because those regions of the borehole which are suspect as a result of irregular logging tool movement are labeled. Indeed, if quality indications suggest poor quality formation data, that portion of the well borehole can be relogged.
The present disclosure sets forth a method and apparatus for making measurements of cable lengths between the surface and the logging tool supported on the wireline, and in particular enables such measurements to be made and corrected for stretch to obtain a corrected depth. The corrected depth is also provided to a recorder. The signal from the logging tool is likewise provided to a recorder. The recorder optionally additionally records cable tension, tool speed, depth correction and also newly provides a quality indicator as a function of depth to enable more critical evaluation of the data from the measurement tool. The present apparatus is utilized with a magnetic mark detector which detects the passage of magnetic markings on the wireline. Moreover, cable tension is continuously measured. In addition opposing calibrated encoder wheels measure cable movement. All of this data is provided to a system which converts the measurements into depth correction, cable tension, tool speed, corrected depth forms output signals supplied to a recorder. The measurement device included in the logging tool also provides an output signal which is conveyed along the wireline to the signal conditioner for recording as a function of depth.